<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title>高度塌陷问题</title>

		<style type="text/css">
			#a {
				background: red;
				zoom: 1;
			}

			#a:after {
				display: block;
				content: '.';
				clear: both;
				line-height: 0;
				visibility: hidden;
			}

			#a1,
			#a2 {
				width: 100px;
				height: 50px;
				margin: 10px;
				background: blue;
				float: left;
			}
			
		   .box{
			   width: 100px;
			   height: 50px;
			   margin: 10px;
			   background: blue;
			   float: left;
		   }
		</style>
	</head>
	<body>
		<div id="a">
			<div id="a1"></div>
			<div id="a2"></div>
			<div class="box"></div>
			<div class="box"></div>
			<div class="box"></div>
			<div class="box"></div>
			<div class="box"></div>
			<div class="box"></div>
			<div class="box"></div>
			<div class="box"></div>
			<div class="box"></div>
			<div class="box"></div>
			<div class="box"></div>
			<!-- <div style="clear: both;"></div> -->
		</div>
	</body>
</html>
